We will solve this question using the ratio method.
∵ You can do 45 sit-ups in 3 minutes
∵ The number of sit-ups is 90
→ By using the ratio method
→ Time: sit-ups
→ 3 : 45
→ x : 90
→ By using cross multiplication
∵ x * 45 = 3 * 90
∴ 45x = 270
→ Divide both sides by 45 to find x

∴ You need 6 minutes to do 90 sit-ups